Fusion down-draw is a leading precision technology developed by Corning Incorporated, Corning, N.Y., U.S.A. for making thin glass sheets suitable for use as liquid crystal display (LCD) glass substrates and in other opto-electronic devices. This process is schematically illustrated in FIG. 1. A stream of molten glass is introduced into a forming trough 103 called isopipe with end-caps 105 at both ends and having two side surfaces converging at a line called root 109 via an inlet pipe 101 coupled to the trough of the isopipe. The glass melt is then allowed to overflow both top surfaces of the trough of the isopipe called weirs, flow down along both side surfaces of the isopipe as two molten glass ribbons 107, then join and fuse at the root 109 to form a single glass ribbon 111, which is then drawn down in the direction 113 and cooled below the root to form the glass sheet with desired dimensions. In the zone below the root, the glass ribbon travels substantially vertically downward while being drawn and cooled from a viscous state, to visco-elastic and eventually to substantially elastic. The elastic glass ribbon is then cut into individual glass sheets, subjected to further finishing such as edge rounding and polishing, and then packaged and shipped to LCD panel makers for use as TFT or color filter substrates. Cutting of the glass ribbon at below the isopipe typically involves the scoring of the ribbon surface, followed by bending along the score-line, whereby discrete glass sheets are separated from the ribbon and then transferred to subsequent steps.
One of the advantages of the fusion down-draw process for making glass sheets is that the surface quality of the glass sheets is high because the quality areas thereof were only formed in the atmosphere and never touched a solid material such as the forming equipment. This process has been used successfully for making glass sheets having a width as large as 3000 mm and a thickness of about 0.6 mm.
The average size of LCDs for the consumer electronics market has increased steadily in the past decade, along with the demand for higher image quality. These have fueled the demand of large-width glass substrates and posed increasingly more stringent requirements for glass sheet quality, such as edge warp and waviness, sheet warp, surface waviness and roughness, thickness uniformity, as well as stress.
At the center of the overflow down-draw process is the isopipe. The dimension and dimension stability of the isopipe has significant impact on the dimension and dimension stability of the glass sheet formed. The isopipe is typically made of a refractory block of material such as zircon ceramics. While the isopipe is supported on both ends, it is typically not supported in the middle. At the high operating temperatures and under the heavy load of the gravity of the isopipe and the glass melt inside the trough and on the surfaces, the isopipe is subject to slow deformation due to a physical phenomenon called creeping. The higher the creep rate of the material of the isopipe, the more the isopipe can creep over a given period of time. In addition, the isopipe material is desirably stable and corrosion-resistance with respect to the glass melt it handles. While zircon was found acceptable for making LCD glass substrates for smaller generation glass sheets, it has relatively high creep rate for even larger generation isopipes, such as those having a length of over 3000 mm. In addition, zircon was found to be less than ideal in corrosion-resistance for some glass compositions.
YPO4-based ceramic materials were proposed for isopipes previously. However, making large-size ceramic materials based on YPO4 is not an easy undertaking. Therefore there remains a need of a large ceramic block based on YPO4 suitable for an isopipe and method for making the same. The present invention satisfies this and other needs.